Conventional center posts of collapsible umbrellas, which are generally constituted by several tubular sections telescopically received within each other, comprise a holding tab which is controlled by an arcuated spring strip and thus having quite a length. The length of each of the sections which constitutes the center post of the collapsible umbrella will be reduced if the number of the sections is increased and in the case of shorter sections, the long holding tab structure is not suitable to be mounted in a single section of the center post. In another known center post structure, a U-shaped spring strip which has a retaining member mounted on one end thereof comprises a loosening prevention device for retaining the sections of the center post in a desired position. This structure is not only more difficult to manufacture but also provides a poor holding capability thereof. Further, the sections of the center post use outward-projected means to hold the sections in a desired relationship so that if the design of the retaining member is not appropriate, the member may either provide a very poor support to the sections or negatively affect the telescoping movement of the sections and thus making the sections of the center post shaking with respect to each other. Taiwan patent application No. 78202620 provides an improvement to overcome the deficiencies of the above prior art center post structures, as shown in FIGS. 6 and 7. The Taiwan patent application uses a U-shaped spring in the holding tab structure with the tab mounted on one end of the spring to reduce the overall length of the holding tab structure. The Taiwan patent application also uses a U-shaped spring with two retaining members respectively mounted on the two ends thereof which are received in two holes formed on the center post in order to support the center post sections with respect to each other in two opposite sides and thus increasing the holding capability thereof. However, since the retaining members are moved in a swinging manner so that the non-linear motion thereof requires an increase in the size of the holes in which the retaining members are received in order to provide a smooth relative movement therebetween. In that case, there are gaps eft between the retaining members and the holes and thus the sections of the center post are not soundly held together. Further, the U-shaped spring should be manufactured and mounted in a very precise manner which leads in an inconvenience in manufacturing. Besides, the use of a U-shaped spring to bias the two retaining members indicates that the retaining members should be exactly 180 degrees opposite to each other and no other relative angular arrangement is possible. All these reveal that it is desirable to have a center post which is further improved to overcome the problems of the prior art.